While it is not within the province of this report to suggest the proper
interpretation of results obtained by the use of the methods herein
specified as standard, the committee feels that a word of caution should
be given regarding the significance of the presence in a water of
members of the B. coli group as defined in this report. Recent work
seems to indicate that the B. coli group as herein defined consists of
organisms of both fecal and non-fecal origin. Therefore care must be
exercised in judging the sanitary quality of a water solely from the
determination of the presence of members of the group.
11. DIFFERENTIATION OF FECAL FROM NON-FECAL MEMBERS OF THE B. COLI
GROUP.
(1) At least 10 cultures should be used. If possible these should be
subcultured from plates made direct from the water since all of the
cultures obtained by plating from fermentation tubes may be descendants
of a single cell in the water. If cultures from water plates are not
available those obtained from plates made as prescribed under B (p. 101)
may be used.
(2) Inoculate each culture into dextrose potassium phosphate broth,[H]
adonite broth, and gelatin. For additional confirmatory evidence
inoculation may be made into tryptophane broth,[I] and saccharose broth.
The dextrose broth must be incubated at 30°. Other sugar broths may be
incubated at 30° or 37° as convenient. Gelatin should be incubated at
20°.
Footnote H:
(a) _Peptone Medium for the Methyl Red Test. To Make One Liter._
1. To 800 cc. of distilled water add 5 grams of Proteose-Peptone,
Difco., or Witte’s Peptone (other peptones should not be substituted),
5 grams c. p. dextrose, and 5 grams dipotassium hydrogen phosphate
(K_{2}HPO_{4}). A dilute solution of the K_{2}HPO_{4} should give a
distinct pink with phenolphthalein.